Influence of Severe Thermomechanical Treatment on Formation of Nanocrystalline Structure in Ni 718 and Ni 718Plus Alloys and their Mechanical Properties

Abstract

The influence of severe thermomechanical treatment via multiple forging on the
formation of a nanocrystalline (NC) structure in bulk samples of Alloy 718 and ATI 718Plus has
been investigated. It was observed that a step-wise decrease of processing temperature from 950
down to 575°C allowed the refinement of the initial coarse grain structure to a NC state.
Investigations of structural changes in the deformed samples have shown that extending the
temperature interval of dynamic recrystallization to low homologous temperatures resulted in the
formation of a fine-grained recrystallized structure. The temperature of NC structure formation in
ATI Alloy 718Plus was 50-100°С higher than that required for alloy 718. This was due to the
presence of the additional γ′-phase, which increased the recrystallization temperature. This
decreased the total strain required to produce NC structure, as compared with Alloy 718. It was
observed that increasing the total strain and decreasing temperatures step-wise during deformation
via multiple forging resulted in a uniform structure across the cross-section of the samples. The
room temperature mechanical properties of the investigated alloys with various grain sizes are also
compared.